Apolipoprotein E (apoE) is essential for the transport and metabolism of lipids in the CNS. Of the three apoE isoforms (E2, E3 and E4), E4 is a major risk factor for Alzheimer's disease (AD). Under normal conditions, it is recognized that the highly specialized mature neurons outsource apoE production to astrocytes, but the mechanism for apoE delivery to neurons remains unclear. Recently, a brain-wide pathway that facilitates convective flow of interstitial fluid (ISF) was shown to play a major role in clearance of by-products of neural activity, including amyloid-beta. This perivascular system of cerebrospinal fluid (CSF) channels-termed the glymphatic system- allows solute exchange with parenchymal ISF by three main pathways: (1) influx of CSF via the peri-arterial space, (2) astrocytic aquaporin 4 (AQP4)-dependent convective flow of ISF through the brain parenchyma, and (3) efflux via para-venous clearance. We propose to test the idea that the glymphatic system also serves as a brain-wide distribution path for the delivery of essential substances to neurons. We choose to test this concept for apoE, since brain apoE is independent of peripheral apoE, CSF could be a source of apoE and because lipid dysregulation is a major risk factor for AD. Our pilot data show, (1) that lipidated apoE (lip-apoE) was circulated by the glymphatic system and entered brain ISF via peri-arterial influx; (2) unexpectedly, fluorescently-tagged lip-apoE was distributed in an isoform specific pattern of zones around penetrating arteries (E4< E3 < E2), and (3) glymphatic distribution of apoE was sharply reduced in aging and in Aqp4-/- mice. We hypothesize that the glymphatic system is critical for brain-wide distribution of apoE to neurons and that age-related failure of this system deprives neurons, located distant to the arteries, access to cholesterol and essential lipids needed for formation and maintenance of synapses. We propose to generate a basic analysis on this novel brain-wide pathway of apoE distribution by an experimental approach centered on the following objectives: Aim 1 will characterize the macroscopic delivery and distribution of apoE isoforms. Aim 2 will assess the effect of aging and AQP4 on the macroscopic distribution of apoE isoforms in conditional inducible Aqp4-/- and control mice. Aim 3 will determine the role of apoE receptors in the macroscopic apoE delivery using specific receptor blocking antibodies. We hypothesize that apoE receptors trap apoE with different efficacy dependent on the apoE isoforms, resulting in the differential distribution profiles, and that reduced peri-arterial convective CSF influx in aging and Aqp4-/- animals will reduce apoE distribution. The implication of the proposed concept is that failure of brain-wide glymphatic distribution of apoE contributes to apoE isoform specific related disorders, including cerebral amyloid angiopathy (CAA) and AD. Our hope is that these studies will generate entirely novel targets for slowing or even preventing AD related neurocognitive decline by improving the glymphatic distribution of apoE, a target that has been ignored so far.